We wish to report the
achievement of two important milestones in our
program to develop tantalum-based superconducting tunnel junction (STJ)
x-ray detectors for use in synchrotron-based XAS spectrometers.

First, by improving grounding of instrumentation and by adding
shielding to cables and electronics enclosures, we have greatly reduced
the severe noise that corrupted the early IV data sets taken on
junctions that were cooled in the cryostat at LLNL and read out with
SFSU electronics.

We have nearly completed a set of beta-Ta STJ devices. Beta-Ta is the
structural phase that is produced in sputtered tantalum films if a
niobium seed layer or other means is not used to nucleate alpha-Ta.
Alpha-Ta is necessary for x-ray detection purposes because it has a
higher critical temperature than the aluminum trapping layer. If
our
latest beta-Ta devices also have excellent IV curves similar to those
obtained for our niobium devices, then it is likely that the shunt
conductance that characterized our original alpha-Ta STJs was not due
to film stress in the tantalum layers but was rather a result of a
small admixture of beta-phase Ta in the nominally alpha-phase Ta
layers. During cool down residual magnetic flux would have been trapped
in the beta-phase regions and perhaps produced NIN regions in what was
supposed to have been an SIS junction.

We have several procedures in mind for rapidly identifying and
correcting the fabrication steps that would produce a mixture of alpha
and beta Ta.